Process for removing tar-forming high molecular weight hydrocarbons from gas mixture

ABSTRACT

Method for production of iron carbide while removing tar-forming hydrocarbons from effluent gas mixtures from Chemical Vapor Deposition or Chemical Vapor Infiltration processes. Method includes passing effluent gas mixture containing hydrogen, methane, and high molecular weight hydrocarbons through a bed that contains iron pellets at elevated temperature, thereby decomposing tar-forming high molecular weight hydrocarbons in the effluent gas mixture and forming iron carbide. The process can be used to clean up effluent streams and/or for carbon sequestration. Apparatus including a de-tarring vessel ( 5 ) having a packed bed ( 7, 8, 9 ) of iron or iron oxide pellets ( 1 ) resting over a perforated distributor plate ( 2 ) and having an exhaust port ( 12 ), the de-tarring vessel being operatively linked via an exhaust port ( 6 ) to a CVI or CVD reactor vessel.

This is a divisional of application Ser. No. 10/932,672, filed Apr. 27,2004, the disclosure of which is incorporated herein by reference.Applicants claim the benefit under 35 U.S.C. §120 of the filing date ofapplication Ser. No. 10/832,672.

FIELD OF THE INVENTION

This invention relates to the production of iron carbide by making useof effluent gas mixtures from industrial processes such as those knownas Chemical Vapor Infiltration and Chemical Vapor Deposition. In thisprocess, effluent gases containing hydrogen, methane, and high molecularweight hydrocarbons are passed through a bed that contains iron pelletsat an elevated temperature in order to convert a portion of the iron inthe pellets into iron carbide and to remove tar-forming high molecularweight hydrocarbons from the gas mixture.

BACKGROUND OF THE INVENTION

The manufacture of carbon-carbon products, in particular aircraftbrakes, involves the deposition of carbon from the decomposition ofhydrocarbon gases at high temperatures (e.g., about 1000° C.) and lowpressures (e.g., about 0.01 atm absolute). Typically, the hydrocarbongas is primarily methane. To enhance deposition rates, however, smallamounts of heavier hydrocarbon gases are commonly included in theprocess.

The deposition of carbon in the manufacture of carbon-carbon compositesis generally accomplished by Chemical Vapor Infiltration (CVI) orChemical Vapor Deposition (CVD). For the purposes of the presentinvention, those processes may be regarded as interchangeable. Gas phaseintermediates are formed from methane in these processes. Those skilledin the art generally agree that the gas phase intermediates formed frommethane are acetylene, benzene, and naphthalene, e.g., by the reaction6CH₄→C₆H₆ (i.e., benzene) and 12H₂Kinetic studies indicate that acetylene is formed first, then acetylenemolecules combine to form benzene, and finally naphthalene is formed.The cyclic hydrocarbon molecules, benzene and naphthalene, are theprecursors for carbon deposition. In the effluent gas, they act as tarformers. They continue to react and grow into higher molecular weightspecies that deposit on surfaces in the downstream gas processingequipment. Such tars represent a great expense in terms of vacuum pumpmaintenance, hazardous waste disposal, and so on. Tar traps have provento be inefficient, and in any case do not solve the hazardous wastedisposal problem. Moreover, even when tar traps are employed, there arestill enough heavy hydrocarbons in the effluent gas to cause problemswith pumps and combustion turbines, etc.

PRIOR ART. U.S. Pat. No. 6,270,741 B1 refers to a conventional methodfor producing iron carbide, in which fine-sized iron ore is charged intoa fluidized bed reactor and reacted with a gas mixture that includes areducing gas such as hydrogen and a carburizing gas such as methane.This reduces and carburizes iron oxides such as hematite (Fe₂O₃) andmagnetite (Fe₃O₄) in a single process. For example,3Fe₂O₃+5H₂+2CH₄→2Fe₃C+9H₂O. U.S. Pat. No. 6,328,946 B1 discloses atwo-step process for producing iron carbide. In the first step, a feedmaterial containing iron materials is contacted with a gas that containshydrogen to produce an intermediate product containing metallic iron. Inthe second step, the intermediate product is contacted with a gas thatincludes (a) carbon monoxide and/or carbon dioxide and (b) hydrogen andoptionally (c) methane. U.S. Pat. No. 6,428,763 B1 discloses a two-stepprocess for producing iron carbide from an iron oxide-containing feedmaterial. In the first step, a feed material containing iron oxide isconverted to an intermediate product by contact with a reducing gas, andin the second step (carburization) step, the metallic iron intermediateis converted into an iron carbide product. The carburizing gaspreferably contains no more than about 5 mole-% carbon dioxide, no morethan about 15 mole-% methane, no more than about 10 mole-% water vapor,and no more than about 10 mole-% inert gases.

SUMMARY OF THE INVENTION

This invention removes tar forming gases from CVD/CVI furnace effluentgas streams. The process can be used to clean up effluent streams and/orfor carbon sequestration. In addition to solving a major processingproblem (that is, tar in the effluent gases), this invention produces auseful commodity (iron carbide) as a by-product. Iron carbide may beused, for instance, as a starting material in the manufacture of steelin mini-steel mills.

One aspect of the present invention is a process for removingtar-forming hydrocarbons from an effluent gas mixture. This processcomprises the step of passing at an elevated temperature (e.g.,400-1100° C., preferably about 600° C.) an effluent gas mixturecontaining hydrogen, methane, and high molecular weight hydrocarbons,through a bed that contains iron pellets, thereby decomposingtar-forming high molecular weight hydrocarbons in said gas mixture. Theeffluent gas mixture is an effluent from a Chemical Vapor Deposition(CVD) process or from a Chemical Vapor Infiltration (CVI) process, suchas a CVD or CVI process that deposits carbon in a fibrous matrixcomprising, for instance, pitch and/or polyacrylonitrile derived carbonfibers. The gas mixture may contain 25 or less weight-% hydrogen, 50 orless weight-% methane, and 3 to 30 weight-% tar-forming high molecularweight hydrocarbons, and a ratio of hydrogen gas to carbon in the gasmixture may range from 2:1 through 5:1. It is most preferred that thecarbonized fibrous matrix is a carbon-carbon composite configured as anaircraft brake disc.

The iron pellets used in the process of this invention generallycomprise iron oxide or metallic iron. The diameters of the iron pelletsrange from 1 to 10 centimeters, preferably from 1 to 4 centimeters.

The process of this invention may include a further step of passing, ata similar elevated temperature, the gas mixture in which tar-forminghigh molecular weight hydrocarbons have been decomposed through a secondbed that contains iron pellets, thereby decomposing tar-forming highmolecular weight hydrocarbons remaining in said gas mixture. In onevariant of this aspect of the invention, the first bed may be locatedinside the CVI reactor, between the carbon-carbon working zone and theexhaust port.

Another aspect of the present invention is an apparatus for de-tarringeffluent gases from a Chemical Vapor Deposition or Chemical VaporInfiltration process. This apparatus includes a de-tarring vesselcomprising a packed bed of iron or iron oxide pellets resting over aperforated distributor plate and having an exhaust port. The de-tarringvessel is operatively linked via an exhaust port to a CVI or CVD reactorvessel. The apparatus may also include a baffle plate and/or a weir. TheCVI or CVD reactor vessel portion of the apparatus may includewater-cooled jacketing and exhaust port insulation.

Yet another aspect of the present invention is a method of fueling a gasturbine which comprises the steps of: carrying out a CVD or CVI process,for instance as a step in the manufacture of an aircraft brake disc;directing effluent gas from said process through an iron oxide pelletbed, wherein said effluent gas is passed through said bed at an elevatedtemperature and wherein a portion of the iron in said pellets isconverted into iron carbide; leading a gas mixture which has passedthrough said bed into a fuel supply inlet of a gas turbine.

Finally, this invention provides a process for the production of ironcarbide which comprises the step of passing a CVD/CVI effluent gasmixture containing hydrogen, methane, and high molecular weighthydrocarbons through a bed that contains iron pellets at an elevatedtemperature in order to convert a portion of the iron in said pelletsinto iron carbide while removing tar-forming high molecular weighthydrocarbons from said gas mixture.

BRIEF DESCRIPTION OF THE DRAWINGS

Various objects and features of the present invention will becomeapparent form the detailed description hereinbelow, considered inconjunction with the accompanying drawings. It is to be understood,however, that the drawings are designed solely for purposes ofillustration and not as a definition of the limits of the invention. Itshould be further understood that the drawings are not necessarily drawnto scale and that, unless otherwise indicated, they are merely intendedto conceptually illustrate the structures and procedures describedherein.

FIG. 1 is a cutaway side view illustrating the apparatus of thisinvention.

FIG. 2 shows reactant profiles at three different times aftercommencement of operation of the process of this invention.

FIG. 3 is a process flow diagram illustrating the production ofsequestered carbon in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The “tar forming gas species” or “tar formers” referred to in thisapplication include acetylene, benzene, and naphthalene. The removal oftar formers from the effluent gas stream is referred to herein as“de-tarring”. Iron ore and iron oxide may be used interchangeably inthis invention. Iron ore pellets typically contain 95 weight-% ironoxide (Fe₂O₃) and 5 weight-% gangue. “Gangue” is the non-valuableminerals or rock associated with an ores in this case, primarily silicaand alumina. Gangue is inert under the processing conditions of thisinvention.

The chemical reactions that produce iron carbide from iron oxide and thehydrocarbon gases in an effluent gas stream from a CVD/CVI reactor inaccordance with this invention are:3Fe₂O₃+11CH₄→2Fe₃C+9CO+22H₂6Fe₂O₃+11C₂H₂→4Fe₃C+18CO+11H₂18Fe₂O₃+11C₆H₆→12Fe₃C+54CO+33H₂30Fe₂O₃+11C₁₀H₈→20Fe₃C+90CO+44H₂

Directly reduced iron ore pellets or sponge iron can be used to striptar formers in accordance with this invention also. This reaction is:3xFe+C_(x)H_(y)→xFe₃C+y/2H₂where, for a tar forming species, x is greater than or equal to y.Complete removal of carbon containing gases from the gas stream producesa pure gas stream of hydrogen that can be used in the “hydrogeneconomy”. Or, the hydrogen can be used to reduce iron oxides to producethe metallic iron used in the above reaction. In that case, the ratio oftotal hydrogen-containing gases to carbon-containing gases leaving theCVD/CVI furnace must be 4.5 to 1, as determined by the stoichiometry of:3Fe₂O₃+2C+9H₂→2Fe₃C+9H₂Owhere C is the total carbon in effluent gases as hydrocarbons and H₂ isthe total hydrogen as hydrocarbons and hydrogen. This embodiment yieldsonly iron carbide and water as products, and no combustible gases forpower generation.

FIG. 1 reference numerals:

-   -   1 iron or iron oxide pellet    -   2 perforated distributor plate    -   3 gases to be used for CVI processing    -   4 gases that have been used for CVI processing    -   5 de-tarring vessel    -   6 CVI exhaust port    -   7 lower portion of packed bed    -   8 central portion of packed bed    -   9 upper portion of packed bed    -   10 weir    -   11 gases that have been de-tarred    -   12 de-tarring vessel exhaust port    -   13 baffle plate    -   14 water-cooled jacketing    -   15 insulation.

FIG. 1. FIG. 1 is a cross-sectional schematic view which illustrates achemical vapor infiltration reactor, is modified to force the effluentgas 4 which contains tar-forming species to pass through a packed bed ofiron ore and/or iron oxide and/or metallic iron. Conventional CVIreactors produce carbon-carbon products by rapidly passing natural gasand/or other hydrocarbon gases 3 through an electrically heated hotzone. Some of the carbon in the incoming gas ends up being depositedwithin the carbon fiber preform matrices located in the CVI reactor. Theeffluent gas therefore (still) contains methane, hydrogen, and tarformers.

De-tarring vessel 5 may be fabricated from steel and may berefractory-lined for thermal insulation. The de-tarring vessel ismounted close to the CVI reactor to lessen the time for the gases toreach the bed to minimize cooling and gas-phase reaction time. Thepacked bed of iron ore is supported by a distributor plate 2 with holesor openings that allow the passage of gas. The effluent gases 4 passthrough the packed bed and exhaust through port 12. Gas process stream11 is constituted by effluent gases 4 after they have been stripped oftar-forming gas species. Schematic slip streams for gases flowing into,through and out of the de-tarring vessel are shown with dashed-linearrows in FIG. 1.

If iron dust were to fall into the CVI reactor, the quality of thecarbon deposition process would be jeopardized. To preventiron-containing material from falling back into the reactor, a baffleplate 13 and a weir 10 are situated near the exhaust port form the CVDreactor. It is important also that the design of CVI exhaust port 6prevents the back-diffussion of carbon monoxide into the CVI reactor.This may be done in two ways. One is to maximize the linear velocity inthe exhaust port by minimizing the diameter of the exhaust port. Thesecond is to make the length of the exhaust port as long as possible.Together, these two design parameters permit the exhaust gas tooverpower the molecular diffusivity of carbon monoxide in hydrogen andmethane. A conflicting criterion is that these design parametersincrease the pressure drop.

Conventionally, CVI reactors have water-cooled shells 15, includingtheir exhaust ports 6. Preferably, the gas path from the working hotzone should be insulated, so as not to cool the gas effluent 4. In FIG.1, this insulation is identified by reference numeral 16. Heat may besupplied to the packed bed to facilitate the reduction and carbiding ofthe iron. Operable temperatures are those for conventional gaseous ironore reduction processes. Temperatures ranging from 400° C. to 1100° C.may conveniently be used, with temperatures of approximately 600° C.being currently preferred.

Because the CVI system operates under a vacuum maintained by downstreampumps, the pressure drop across the packed bed and distributor platemust be less than the operating pressure required in the CVI reactor.The pressure drop should preferably be less than about 2 torr. Thede-tarring vessel may be operated between 0.001 atm to about 1 atm,depending on the operating pressure of the CVI reactor. There are threeconvenient ways to achieve a low pressure drop. One is the use of largediameter pellets. Large pellets reduce the amount of frictional forcethat the fluid exerts on the surface of the pellets. Pellet diameters of1 to 10 centimeters are preferred, with pellets having diameters up to 4centimeters being especially preferred. The second way to achieve thelow pressure drop is to use a low linear velocity of the gas through thebed. This also reduces the frictional drag of the gases on the pellets.To lower the velocity, a large diameter de-tarring vessel may be used.The third expedient for achieving low pressure drop is to make thepacked bed as shallow as possible. These variables must, of course, bebalanced by the requirement that the de-tarring vessel needs to holdenough solids to capture as much as possible of the tar forming speciesfor the duration of at least one CVI processing cycle.

The use of a fluidized bed reactor for scrubbing tar formers from theeffluent gas is possible. However, with this approach, fine particulatesof iron ore must be used to meet the low pressure drop requirement.Also, fluid beds involve grinding and dust generation. Entrained dustcauses excess wear on vacuum pumps.

DE-TARRING. The bed packed with iron ore pellets is heated at the sametime as the CVI reactor, with some of the heat coming from the CVIreactor effluent gas. Once processing gas 3 starts to flow, the firstgas to reach the packed bed contacts fresh iron oxide. Thecarbon-bearing gas species proceed to react with the surface oxide asper the following reactions:3Fe₂O₃+11CH₄→2Fe₃C+9CO+22H₂6Fe₂O₃+11C₂H₂→4Fe₃C+18CO+11H₂18Fe₂O₃+11C₆H₆→12Fe₃C+54CO+33H₂30Fe₂O₃+11C₁₀H₈→20Fe₃C+90CO+44H₂

Once the surface is carbided, the process gases diffuse into theinterior of the ore pellet through micro-channels. However, the fastestdiffusing molecule in this chemical system is hydrogen, so the interioroxide will be reduced to sub-oxides and metallic iron before it iscarbided. Reduction of iron oxide with hydrogen is well known:3Fe₂O₃+H₂→2Fe₃O₄+11H₂OFe₃O₄+4H₂→3Fe+4H₂O

At temperatures above 600° C., FeO is present in the system, but it isomitted here for purposes of simplicity. The diffusion of water from thepellet's interior will inhibit carbiding by locally raising the oxygenpotential on the pellet's surface. Water vapor entering the gas streamwill destroy tar forming molecules, for example:6H₂O+C₆H₆→6CO+9H₂

FIG. 2. During a carbon-carbon processing cycle, the progression ofreaction of the packed bed or iron ore will proceed as schematicallyillustrated in FIG. 2. The three plots shown in FIG. 2 representincreasingly longer elapsed times, with the top drawing (time=1) beingshortly after the start of reaction. The middle and bottom drawingsrepresent later stages in the reaction, with “time=3” being near the endof the useful life of the packed bed. The unreacted bed of hematite(Fe₂O₃) is first converted to magnetite (Fe₃O₄) by the reaction:3Fe₂O₃+H₂→2Fe₃O₄+H₂O

As time proceeds, more and more of the packed bed is converted, and anS-shaped concentration profile, or reaction wave, will move toward theexhaust end of the bed (the top in FIG. 1). Simultaneously, the freshincoming effluent gas reduces the magnetite to metallic iron by thereaction:Fe₃O₄+4H₂→3Fe+4H₂OThat “reaction wave” also moves through the bed, following the hematiteto magnetite wave. Finally, the metallic iron is carbided by thereaction:3xFe+C_(x)H_(y)→xFe₃C+y/2H₂

The exact shape of the “waves” depends on the reactivity of the system.Very rapid reactions will have more of a reaction “wall” shape withsmall “tails”. Slow reactions will set up flat “waves”. The de-tarringreactors in accordance with the present invention will generally haveslow reaction kinetics, due to their use of large diameter particles,low temperatures, and high gas velocities. As the packed bed becomesfully carbided, unreacted gas will break through the bed. In accordancewith this invention, gas treatment will generally be halted before thegas breaks through. The compositional profile at the end of acarbon-carbon processing cycle in accordance with this invention willresemble that shown at the bottom of FIG. 2 (time=3).

Returning to FIG. 1, at the end of the CVI processing cycle, the lowerpart of the packed bed 7 will be carbided iron (Fe₃C). The midsection 8of the packed bed will be mostly metallic iron (Fe). The top portion 9of the packed bed would still be primarily magnetite (Fe₃O₄). When thepellets are removed, the carbide pellets are removed (and sold), whilethe rest of the pellets will be reused, along with added fresh iron oreor oxide pellets. Pellet manipulations may be accomplished in a varietyof ways. One approach is to have two distinct de-tarring reactorsstacked, such that the gases pass through the lower vessel first, thenthrough the upper one. When the iron carbide is removed from the lowervessel, the pellets in the upper vessel are moved to the lower vesseland fresh ore is charged into the upper vessel. In a variant of thisapproach, the lower packed bed is located inside the CVI reactor,between the carbon-carbon working zone and the exhaust port 6. In thisvariant, fully reduced iron pellets are preferred for placement insidethe CVI reaction, in order to prevent the formation of carbon monoxidegas near the product being treated.

Another option for this invention is the use of directly reduced ironinstead of iron ore. This has the advantage of completely removing thecarbon from the gas stream to produce a hydrogen-only gas stream. Thisseparates the carbon for eventual sequestration and generates hydrogenthat can be used, for instance, in hydrogen fuel cells. The use ofdirectly reduced iron simplifies the handling of the bed pellets, sincethere will be no oxide-containing pellets.

EXAMPLES

Tables IIa and IIb show Examples A, B, C, and D. Examples A and B useiron ore pellets. Examples C and D use metallic iron pellets. TABLE IIaExample A B Packed Bed properties Iron type Iron oxide Iron oxideTemperature 600° C. 700° C. CVI Reactor effluent concentration, weight-%Methane 78.6 55.7 Molecular hydrogen 7.4 15.7 Tar formers 14.0 28.6 H₂/Cmolar ratio 2.3 2.8 Pressure 0.01 atm 0.01 atm Equilibrium results Ironcarbide produced per effluent Reduction in tar formers gas treated,weight basis 50% 0.20 0.40 95% 0.45 0.90 Concentration of carbonmonoxide in Reduction in tar formers product stream, volume-% 50%  5% 7% 95% 11% 15% Reduction in tar formers CH₄/H₂ molar ratio in productstream 50% 1.16 0.40 95% 0.93 0.29

TABLE IIb Example C D Packed Bed properties Iron type Metallic MetallicTemperature 600° C. 700° C. CVI Reactor effluent concentration, weight-%Methane 78.6 71 Molecular hydrogen 7.4 25 Tar formers 14.0 4 H₂/C molarratio 2.3 4.6 Pressure 0.01 atm 0.01 atm Equilibrium results Ironcarbide produced per effluent Reduction in tar formers gas treated,weight basis 50% 1.0 0.67 95% 2.4 2.6 Reduction in tar formers CH₄/H₂molar ratio in product stream 50% 1.25 0.32 95% 1.00 0.22

DISCUSSION. Examples A and C have the same gas composition. Examples Band D have high and low levels of tar formers, respectively. Example Duses all effluent gas with a high hydrogen content. The equilibriumcalculations show that the amount of carbide produced when 50% of thetar formers are removed from the gas stream varies from 0.2 to 1.0 kg ofcarbide per kg of treated gas. More tar-forming gases are consumed whenoxides are carbided than when elemental iron is carbided. Much of thecarbon in the tar-forming molecules is consumed to produce carbonmonoxide. Example B, with twice the tar-forming species as Example A,produces twice the amount of iron carbide. For 95% removal of tarformers, both Examples C and D produce about 2.5 tons of carbide per tonof effluent treated. However because the starting levels of tar formersare significantly different for these two examples, the absolute levelsof tar formers in the product gases are correspondingly different. Inpractice, an important goal may often focus on achieving acceptablemaximum amounts of tar formers left in the effluent gas, rather thanfocusing on the percentage of tar formers removed from the gas. Asdemonstrated by the above Examples, this invention may be carried out toachieve a wide range of percentage reductions in tar formers. Thisaspect of the invention accordingly enables achievement of a wide rangeof desired maximum amounts of tar formers in CVD/CVI effluent gases.

To achieve a 95% reduction in tar formers, more than twice the amount ofiron-containing solids is required compared to that needed for a 50%reduction. The production or iron carbide is linear with reduction intar formers until near complete removal, at which point the removaltails off. The oxygen from the oxide ends up as carbon monoxide—there isessentially no water vapor or carbon dioxide. At 95% removal of tarformers, the level of carbon monoxide reaches 1% and 15% for Example Aand B, respectively, As de-tarring proceeds, the ratio of methane tohydrogen decreases. This is caused not only by the release of hydrogento the gas but also by consumption of methane to reduce oxide.

FIG. 3 features:

-   -   CVD/CVI step    -   de-tarring/carbiding step    -   vacuum pumping    -   ore reduction step    -   hydrogen separation step.

FIG. 3. Complete separation of the carbon-containing gases forsequestration can be achieved by using all of the methane to carbidesponge iron. In FIG. 3, sponge iron is used in the de-tar/carbiderreactor not only to de-tar the effluent gas stream but also tocompletely convert the methane to iron carbide and hydrogen. Table IIIlists mass balances for process streams labeled as 1-9, 11, and 13 inFIG. 3. Examples C and D in Table III are the same as Examples C and Din Table IIb, but in Table III the gas stream is completely“de-carbonized” instead of just having tar formers removed as shown inTable IIb. The hydrogen and carbon are essentially completely separatedinto different streams. Equilibrium thermodynamic calculations indicatethat the concentration of methane in the stream leaving the carbider is0.8% and 0.2% for Examnples C and D, respectively. TABLE III Weight ofcomponents, metric tons Stream number Component Example C Example D 1CH₄, H₂, tar formers 1 1 2 Fe₃C 10.7 8.5 3 H₂ (H₂O) 0.3 0.4 4 Metalliciron 10.0 7.9 5 H₂ (H₂O) — 1.8 (0.04) 6 Fe₂O₃ — 11.4 7 H₂ (H₂O) — 1.4,3.9 8 Water — 3.8 9 H₂ (H₂O) — 1.4 (0.04) 11 H₂ (H₂O) — 0.004 (nil) 13H₂ (H₂O) — 1.4 (0.04) Ore reduction temperature/pressure, C./atm 600/1H₂ separation efficiency of water removal 99.0%

In Example D, pure hydrogen is used to reduce the iron ore, Stream 3 isbrought up to 1 atm or higher pressure through the vacuum pumps 25 andis combined with a recycle stream 13. This is required for completereduction of the ore to metallic iron. For instance, at 600° C. 3.2moles of hydrogen are required to produce 1 mole of water from thereduction of magnetite to metallic iron. At higher temperatures, lessthan 3.2 moles are required, and at lower temperatures, more than 3.2moles are required. The hydrogen may be recovered for recycle using ahydrogen separation unit 40. The separation unit may be, for instance, acondenser or a membrane separation unit.

The effluent gas 1 used in Example C does not contain enough hydrogen toreduce all of that ore that is carbided. Therefore, no weights arelisted in Table III for streams 5 through 9, 11, and 13. Example D useseffluent gas that has sufficient hydrogen to reduce the ore. In fact, inExample D the final product is only iron carbide and water. There iseffectively no hydrogen in exhaust stream 11. For both examples, onemetric ton of effluent gas carbide significant amounts of iron—10 and7.9 metric tons or iron for Examples C and D, respectively.

1. A process for the production of iron carbide which comprises the stepof passing a CVD/CVI effluent gas mixture containing hydrogen, methane,and high molecular weight hydrocarbons through a bed that contains ironpellets at an elevated temperature in order to convert a portion of theiron in said pellets into iron carbide while removing tar-forming highmolecular weight hydrocarbons from said gas mixture.
 2. The process ofclaim 1, wherein said iron pellets comprise iron oxide or metallic iron.3. The process of claim 2, wherein the diameters of said iron pelletsrange from 1 to 10 centimeters.
 4. The process of claim 3, wherein thediameters of said iron pellets range from 1 to 4 centimeters.
 5. Theprocess of claim 1, wherein said gas mixture contains 25 or lessweight-% hydrogen, 50 or less weight-% methane, and 3 to 30 weight-%tar-forming high molecular weight hydrocarbons.
 6. The process of claim5, wherein the ratio of hydrogen gas to carbon in said gas mixtureranges from 2:1 through 5:1.
 7. The process of claim 1, wherein saidelevated temperature is in the range 400° C. through 1100° C.
 8. Theprocess of claim 7, wherein said elevated temperature is approximately600° C.
 9. The process of claim 1, wherein said gas mixture is aneffluent from a Chemical Vapor Deposition process or from a ChemicalVapor Infiltration process.
 10. The process of claim 9, wherein saidChemical Vapor Deposition (CVD) process or Chemical Vapor Infiltration(CVI) process deposits carbon in a fibrous matrix.
 11. The process ofclaim 10, wherein said fibrous matrix comprises pitch and/orpolyacrylonitrile derived carbon fibers.
 12. The process of claim 11,wherein the carbonized fibrous matrix is a carbon-carbon compositeconfigured as an aircraft brake disc.